Jaager

I am not a kit builder, but a cursory inspection of web advertising for the English brig Mantua Golden Star produced a few red flags:
 
The company description defines it as a plank-on-frame kit.  It is plank-on-bulkhead.  A very misleading indicator for its level of sophistication and leads to questions about basic company ethics.
 
From a photo on the Mantua site, I count 8 moulds (bulkheads) - which could pose a problem as an adequate base for planking.
 
The scale is 1:150. 
This is well into the miniature range.  While ship plans can be scaled, the physical properties of wood cannot. 
The relatively wide spacing of the moulds could be mitigated somewhat because the wood will resist the more acute bending.  There will be less of a problem with hollows., There will probably more of a problem with bulging.  Pushing bulges in will be fruitless if there is nothing there to bond the plank to.  Sanding a bulge down may remove too much wood. A layer of scrap wood can be scabbed on the inside between the moulds that will provide the removed substance. 
At 1:150 scale, the replication of smaller detail will require creativity in the choice of materials.  Getting the rigging to scale will be a bear. 
Walnut has open pores. Not as extreme as Oak, but still there.  The Lime surface and grain will scale better in appearance.  The inherent fuzziness and friability will present more of a problem as show material in miniature.
 
A brig is an excellent candidate for modeling at 1:48.  Reasonable in size and open to detail.  It is still not a size that can be pushed to the back of a book shelf though.   A 1:150 brig will not  require much room to display,  but will only be 3.3% of the volume of the 1:48 version.
 

Stee,
Your model does not look to be particularly affected by dust...yet.  The main source for damage and destruction for any wooden ship model is in the failure to place it in a proper case.  They can vary from an elegant piece of wooden furniture with glass or clear plastic sides and top to a glued plastic box.  All choices will involve some cost.  If you wish it to last, a case is pretty much necessary.
Keep it away from direct sunlight and allow for filtered air exchange.
 
It is a worthy icon for your grandfather's memory.  Keeping it and keeping it safe would show great respect.  He spent no little time and energy building it.  If you do case it, attach a narrative with as much information about him and it as you can gather to the bottom.  This way when this is passed on to a later generation,  more than vague and mostly forgotten stories will be with it.

Permanently filling the spaces above the wale added stability.  Consider a keelson;  and bilge riders at the head of futtock 1.   The clamps will certainly secure things.

Stee,
Your model does not look to be particularly affected by dust...yet.  The main source for damage and destruction for any wooden ship model is in the failure to place it in a proper case.  They can vary from an elegant piece of wooden furniture with glass or clear plastic sides and top to a glued plastic box.  All choices will involve some cost.  If you wish it to last, a case is pretty much necessary.
Keep it away from direct sunlight and allow for filtered air exchange.
 
It is a worthy icon for your grandfather's memory.  Keeping it and keeping it safe would show great respect.  He spent no little time and energy building it.  If you do case it, attach a narrative with as much information about him and it as you can gather to the bottom.  This way when this is passed on to a later generation,  more than vague and mostly forgotten stories will be with it.

POF - the framing at higher scales seems to me to require a higher volume of wood than I would anticipate - if I did not know better from experience.  You may come to understand what I meant by "Take out a loan and buy as much as you can."  I hope that I am wrong about this in your case.  If I calculated correctly, you are already $500 invested. The loss to kerf, really hurts.  The waste from scroll cutting individual timbers can approach 50% or more.  Mid ship is a lower loss to waste.  It gets progressively more wasteful as the bow and stern are approached.  If you assemble your frames by gluing up the plank stock in a horse shoe and scroll cutting the whole frame from a pattern on that,  all bets are off on the degree of waste.  That is in the WOW! zone.
If you are not subject to compulsion about uniformity in materials used, it may not be painful for you to be ruthless.  It would save your  supply and your financial reserves if the Castello was reserved for only those regions where it will be on display, and an Acer species is used in the hidden regions. 
If you are one of the meshuga and intend to display the innards at a higher range scale, instead of planking the topside and decks completely,  boy howdy on the volume used there!
Anyway, I admire your optimism in expecting even a Swan class sloop to only cost you one billet.

I think you got it right.

The billets do not appear to be wet enough to worry about fungus.  I have never read of chemical fungicides  being used anywhere around this.  
If the ends are not sealed, do that.  Old paint can be used, but you do not wish to contaminate any of the wood with paint spread,  you can melt a pot of candle wax and dip the ends in it.
 
Definitely sticker the billets  ~ 1/4" gap should do -  If you are OCD,  place stickers on the top layer and place a layer of heavy weight - like concrete building blocks.  Where you store them - have good air flow. 

The billets do not appear to be wet enough to worry about fungus.  I have never read of chemical fungicides  being used anywhere around this.  
If the ends are not sealed, do that.  Old paint can be used, but you do not wish to contaminate any of the wood with paint spread,  you can melt a pot of candle wax and dip the ends in it.
 
Definitely sticker the billets  ~ 1/4" gap should do -  If you are OCD,  place stickers on the top layer and place a layer of heavy weight - like concrete building blocks.  Where you store them - have good air flow. 

The billets do not appear to be wet enough to worry about fungus.  I have never read of chemical fungicides  being used anywhere around this.  
If the ends are not sealed, do that.  Old paint can be used, but you do not wish to contaminate any of the wood with paint spread,  you can melt a pot of candle wax and dip the ends in it.
 
Definitely sticker the billets  ~ 1/4" gap should do -  If you are OCD,  place stickers on the top layer and place a layer of heavy weight - like concrete building blocks.  Where you store them - have good air flow. 

Allan,
Another member brought this up some time ago, apparently the advertised thread count in fabric is not all that straight forward in what it is describing.  I find it confusing, but it may be that 500 or so may be the max available - for what we want it to mean.  If I read it correctly a 1000 count is actually 500 threads, with each thread being two yarns twisted up.  It is probably thicker.  The variety of cotton used - some have longer fibers - and how much fractionation and purification of just the long fibers before twisting up may enter in to it.

Ummm.  You might consider altering the title to: Bulkhead fairing techniques. 
Your present title is bait for POF scratch builders.
Frames are a whole nuther thing.  While they are just assembled frames, they are beveled as an isolate unit.  A sanding drum with 60 grit or 80 grit does an efficient job for me,  Fine tune with 220 grit.  Once they are placed on the keel, the final fairing can get tricky - if the spaces between the frames are not filled with bracing.  Moving the frames because of too much force being applied is not good.  I have the spaces with temporary filler wood, so I can be vigorous about it.
 
The same problem can occur with POB -it seems to me - in theory.  The molds ( bulkheads ) are just hanging out there, if there is not bracing between them.  Then there is the whole alternative of filling between the molds and thereby provide an adequate base for the planking.

OCD? You mean Obsessive-Compulsive Disorder? 🤗 Thank you for all the information. One of the billets will definitely become a POF swan class in the future ..

The billets do not appear to be wet enough to worry about fungus.  I have never read of chemical fungicides  being used anywhere around this.  
If the ends are not sealed, do that.  Old paint can be used, but you do not wish to contaminate any of the wood with paint spread,  you can melt a pot of candle wax and dip the ends in it.
 
Definitely sticker the billets  ~ 1/4" gap should do -  If you are OCD,  place stickers on the top layer and place a layer of heavy weight - like concrete building blocks.  Where you store them - have good air flow. 

Ummm.  You might consider altering the title to: Bulkhead fairing techniques. 
Your present title is bait for POF scratch builders.
Frames are a whole nuther thing.  While they are just assembled frames, they are beveled as an isolate unit.  A sanding drum with 60 grit or 80 grit does an efficient job for me,  Fine tune with 220 grit.  Once they are placed on the keel, the final fairing can get tricky - if the spaces between the frames are not filled with bracing.  Moving the frames because of too much force being applied is not good.  I have the spaces with temporary filler wood, so I can be vigorous about it.
 
The same problem can occur with POB -it seems to me - in theory.  The molds ( bulkheads ) are just hanging out there, if there is not bracing between them.  Then there is the whole alternative of filling between the molds and thereby provide an adequate base for the planking.

Ummm.  You might consider altering the title to: Bulkhead fairing techniques. 
Your present title is bait for POF scratch builders.
Frames are a whole nuther thing.  While they are just assembled frames, they are beveled as an isolate unit.  A sanding drum with 60 grit or 80 grit does an efficient job for me,  Fine tune with 220 grit.  Once they are placed on the keel, the final fairing can get tricky - if the spaces between the frames are not filled with bracing.  Moving the frames because of too much force being applied is not good.  I have the spaces with temporary filler wood, so I can be vigorous about it.
 
The same problem can occur with POB -it seems to me - in theory.  The molds ( bulkheads ) are just hanging out there, if there is not bracing between them.  Then there is the whole alternative of filling between the molds and thereby provide an adequate base for the planking.

Allan,
Another member brought this up some time ago, apparently the advertised thread count in fabric is not all that straight forward in what it is describing.  I find it confusing, but it may be that 500 or so may be the max available - for what we want it to mean.  If I read it correctly a 1000 count is actually 500 threads, with each thread being two yarns twisted up.  It is probably thicker.  The variety of cotton used - some have longer fibers - and how much fractionation and purification of just the long fibers before twisting up may enter in to it.

Allan,
Another member brought this up some time ago, apparently the advertised thread count in fabric is not all that straight forward in what it is describing.  I find it confusing, but it may be that 500 or so may be the max available - for what we want it to mean.  If I read it correctly a 1000 count is actually 500 threads, with each thread being two yarns twisted up.  It is probably thicker.  The variety of cotton used - some have longer fibers - and how much fractionation and purification of just the long fibers before twisting up may enter in to it.

Wahka_est,
It is not so much that as it is the implication that Danish oil is somehow special.  I have used Tung oil for the same purpose on a DIY Gerstner tool chest.  It is that shellac will do what you want.  With less time needed between coats and a better reverse gear.  The depth can be fine tuned - more coats, more effect. 
 
The following are quotes from Google. No attribution, so no way to judge the accuracy.
 
"Danish oil is a wood finishing oil, often made of tung oil or polymerized linseed oil, although there is no defined formulation so its composition varies among manufacturers. ... It is a "long oil" finish, a mixture of oil and varnish, typically around one-third varnish and the rest oil."
"WATCO Danish Oil is a unique blend of penetrating oil & varnish hardens in the wood, not on the wood. Watco Danish Oil penetrates deep into wood pores to protect from within and to enhance the natural look and feel of the wood. It creates the rich, warm glow of a traditional hand-rubbed finish."  This is obviously advertising copy. (seduction and manipulation)
"Danish oil is easier to use than tung oil because it penetrates wood faster and it also dries faster than tung oil. On the other hand, tung oil cures to a very hard and beautiful, golden finish, which is often worth all the trouble of applying it."

Y.T.,
You must first please yourself.  The grain contrast is a personal factor. 
If you are new enough at this to not realize what kit instructions are intended to ( can't find the verb )  as far as adherence to absolute historical accuracy,  it is proper and expected that you follow them. 
If you are seduced further into all this, more knowledge and sophistication will follow.  It is a natural and necessary progression.  One common to all of us who did not graduate from The Webb Institute with a minor in the historical aspects of ship building (if they even have such a thing). The range of what all of this covers is not an impossible to encompass, open ended challenge.  History has set limits.  Loss of information has allowed for creativity and interpretative challenge.  It is broad enough for a lifetime of interest.
 
I would never enter or judge any sort of contest.  So my opinion is not a factor.  The key for me is to pick a goal and objective for just what is wished to be displayed  and do the best to meet those targets.  That has evolved quite a lot over time. 
 
For your next ship,  if historical accuracy is a goal, look up the shift of the butt joints on deck planking that was actually allowed.  
 
There are very generally two schools of thought on deck and hull fastenings:
none,  because for the deck at least, they were covered with plugs of the same species placed with the same grain orientation and were thus invisible from any distance.
Show,  there was a rule about number and stagger pattern.  There was a rule about diameter. They were not just used at the butt joins.
I like them for two reasons. 1. The look.   2.  Their locations can be used with pins and hitch chocks to hold and clamp until the Titebond dries.  Then if brass pins are used, and brass trunnels wanted, nip and file.  If bamboo trunnels, pull, broach, and glue in the bamboo.  The pain is dealing with getting the pattern location correct on the loose plank.
 

What are the sources that even suggest that any sort of penetrating oil is commonly used on a ship model?  A few use it.  The approval and suitability for use on a ship model expressed in the way that you wrote this inquiry probably only exists in the minds of the owners and advertising department of the Watco Co.  Watco = Danish Oil = $
Ship models seldom present a large enough flat surface to need or need to show the effects of depth with a clear finish.
Unless your model is and is meant to be a toy, a high gloss finish is a scale inappropriate look.
Most any water clear oil will give give wood a wet look.  Most oils never dry and ruin the surface for any follow on treatment.  Penetrating oils are a special case . As they are exposed to air, they undergo a chemical reaction (you hope) and cross link (polymerize).  They become a clear "hard, maybe" solid.  What they allow to be bonded to their surface is particular and specific to the type of oil used.
 
Now,  to back up a step or two and address your real problem:
This is opinion, not law or rules:
After you sand the deck, give it a light scraping with a sharp single edged razor blade. Vacuum and then wipe with 95% ethanol.
I do not think you really want a deck on a ship model to express sharpness or contrast in the wood grain used.  The goal is to use a wood species with no visible grain.
Apply a coat of shellac - the primer coat (1st coat) is 50:50  shellac: 95% ethanol  (shellac thinner).
Follow with a coat of full strength shellac.  Shellac just dries.  There is no chemical reaction.  The more layers, the more depth effect. ( French polish )
If anything plastic is not against Your rules, a wipe on polyurethane finish seems to be a popular final finish instead of or over the full strength shellac. But unless yours is a toy, not high gloss.

Bob,
No disagreement from me about any sort of Al oxide - open coat.  For the longest time,  I wondered what the difference was between open coat and closed coat.  I was addressing the closed coat Al oxide and Si carbide and Zirconia as being the stuff that could clog.  I think that the longer lasting open coat is about having a cloth or heavier paper backing and a bonding agent that is stronger and more heat from friction resistant.  The teeth part should be about the same.
 
Dave,
Not having a ton of fine saw dust would be a plus,  but at least needing protection from it,  in addition to a shop vac meant that I had some N-95 masks on hand.

What are the sources that even suggest that any sort of penetrating oil is commonly used on a ship model?  A few use it.  The approval and suitability for use on a ship model expressed in the way that you wrote this inquiry probably only exists in the minds of the owners and advertising department of the Watco Co.  Watco = Danish Oil = $
Ship models seldom present a large enough flat surface to need or need to show the effects of depth with a clear finish.
Unless your model is and is meant to be a toy, a high gloss finish is a scale inappropriate look.
Most any water clear oil will give give wood a wet look.  Most oils never dry and ruin the surface for any follow on treatment.  Penetrating oils are a special case . As they are exposed to air, they undergo a chemical reaction (you hope) and cross link (polymerize).  They become a clear "hard, maybe" solid.  What they allow to be bonded to their surface is particular and specific to the type of oil used.
 
Now,  to back up a step or two and address your real problem:
This is opinion, not law or rules:
After you sand the deck, give it a light scraping with a sharp single edged razor blade. Vacuum and then wipe with 95% ethanol.
I do not think you really want a deck on a ship model to express sharpness or contrast in the wood grain used.  The goal is to use a wood species with no visible grain.
Apply a coat of shellac - the primer coat (1st coat) is 50:50  shellac: 95% ethanol  (shellac thinner).
Follow with a coat of full strength shellac.  Shellac just dries.  There is no chemical reaction.  The more layers, the more depth effect. ( French polish )
If anything plastic is not against Your rules, a wipe on polyurethane finish seems to be a popular final finish instead of or over the full strength shellac. But unless yours is a toy, not high gloss.

Dave,
I think the major alternatives are quarter sawn and plane sawn, yes?
The way I read the Gilmer quote:  their stock is quarter sawn, and the wide face is inside a growth ring.  The theoretical and perfect effect would be no grain showing and all one color.  More or less perfect for scale effect.  The narrow face would be busy with grain and it would be parallel lines. 
 
My stock (primarily Hard Maple and Black Cherry) from Yukon Lumber (they are not familiar with Castello) is plane sawn.  The wide face is along the growth rings. With that face on the saw table, what the sawn slices show is across the growth rings.  The stock for frame timbers has grain effect on the surface that is between each frame.  The edge that shows could, in theory, show no grain,  if the frame was a box and not curved.  It is actually catch as catch can,  because the lumber stock is a slice along the length of a cylinder with concentric rings.  With Hard Maple,  what shows on a cut across the  rings face can be plain, flame, tiger, fleck, depending on ring angle and all from the same board. The edge on a single timber can look as though it was two mildly different colored pieces with a curved diagonal scarph.   Were I to choose to be OCD about this part, I figure it would throw me into a fugue state.  I just live with it.
 
With flatsawn,  the face that you choose to lay on the saw table will have a significant and possibly more predictable and uniform effect on what your ultimate display face will look like. 
 
 
 

What are the sources that even suggest that any sort of penetrating oil is commonly used on a ship model?  A few use it.  The approval and suitability for use on a ship model expressed in the way that you wrote this inquiry probably only exists in the minds of the owners and advertising department of the Watco Co.  Watco = Danish Oil = $
Ship models seldom present a large enough flat surface to need or need to show the effects of depth with a clear finish.
Unless your model is and is meant to be a toy, a high gloss finish is a scale inappropriate look.
Most any water clear oil will give give wood a wet look.  Most oils never dry and ruin the surface for any follow on treatment.  Penetrating oils are a special case . As they are exposed to air, they undergo a chemical reaction (you hope) and cross link (polymerize).  They become a clear "hard, maybe" solid.  What they allow to be bonded to their surface is particular and specific to the type of oil used.
 
Now,  to back up a step or two and address your real problem:
This is opinion, not law or rules:
After you sand the deck, give it a light scraping with a sharp single edged razor blade. Vacuum and then wipe with 95% ethanol.
I do not think you really want a deck on a ship model to express sharpness or contrast in the wood grain used.  The goal is to use a wood species with no visible grain.
Apply a coat of shellac - the primer coat (1st coat) is 50:50  shellac: 95% ethanol  (shellac thinner).
Follow with a coat of full strength shellac.  Shellac just dries.  There is no chemical reaction.  The more layers, the more depth effect. ( French polish )
If anything plastic is not against Your rules, a wipe on polyurethane finish seems to be a popular final finish instead of or over the full strength shellac. But unless yours is a toy, not high gloss.

In thinking about the theory of the thing, it could have a real resistance to solar gain if there was a double roof.  Leave the old trusses and plywood (or MDF) sheathing.  Remove the old shingles - because of their weight.  Add riser blocks to determine the gap and add a new roof with new rafters, sheathing and shingles over it.  Active exhaust of the air in the gap would divert the heat.  Of course the additional weight may crush the walls and the whole thing be a quantum singularity for your budget. 
 
If there was sufficient insulation, a free standing ceramic space heater may be enough for most Winter conditions.  Not shirt sleeve, but not ice sickle.  I was going to try one in my garage, but I finished what I needed to do down there on La Renommee before it got cold.  Then my Black Dog got aholdt of my initiative, so I did not need to buy one.